In the field of power supplies, the use of an LLC converter is known. An LLC converter comprises a series arrangement of a first parallel arrangement of a first switch and a first diode, and a second parallel arrangement of a second switch and a second diode. The series arrangement is coupled between a first input terminal and a second input terminal for receiving a DC input voltage. A voltage on the first terminal is positive with respect to a voltage on the second terminal. The cathode of the first diode and the cathode of the second diode are directed to the first terminal. The first diode may be extrinsic, or may be intrinsic to the first switch. Likewise, the second diode may be extrinsic, or may be intrinsic to the second switch. A series arrangement of a capacitor, a first inductor, and a second inductor is coupled in parallel to either the first diode or the second diode. One of the first inductor and the second inductor may be a transformer. A rectifier and filter are coupled to either the first inductor or the second inductor for supplying a filtered DC output voltage. A control circuit comprises a switching control means for controlling the frequency of an on and off switching of the first switch and the second switch.
The topology of the LLC converter has a number of advantages, such as a low Electromagnetic Interference, EMI, and a high efficiency. The output voltage of the LLC converter normally is controlled by feedback control of the switching frequency of the switches. The LLC converter may be driven above the resonance frequency in order to avoid hard switching. In this so-called soft switching mode, the current through the switched-off first switch is positive just before the moment of switching-off. As a result, the voltage on the connecting node between the first and second switches commutates, and the second diode parallel to the second switch commutates, and the second diode parallel to the second switch starts conducting current. The second switch may be switched on at the moment the second diode is conducting, so virtually no switching losses occur. At such operating conditions, the use of MOSFETs, Metal Oxide Semiconductor Field Effect Transistors, as switches comprising intrinsic diodes is most suitable.
For a stable control behavior close to the resonance frequency of the LLC converter, the control circuit would have to be adapted for every change of operating conditions. However, for most applications this is not a feasible solution. On the other hand, frequency control can be used when the operating frequency is not close to the resonance frequency. However, in this situation the needed frequency sweep would be large in order to cover all input and output voltage variations. Thus, when using a LLC converter, normally extensive (and therefore expensive) control circuitry is necessary to obtain the desired performance, for example when the LLC converter would be used in a power supply circuit for driving an LED lighting module containing a plurality of LEDs, such as one or more strings of LEDs, in one or more color channels.